Preparation of esters of 2-ketoaldonic acids by oxidation of aldonic acids, their lactones and esters



Patented July 16, 1940 UNITED STATES PATENT OFFICE PREPARATION OF ESTERS OF Z-KETO- ALDONIC ACIDS BY OXIDATION OF AL- DONIC ACIDS, THEIR LACTONES AND" ESTERS Richard Pasternack, Brooklyn, N. Y., and Peter P. Regna, North Bergen, N. J., asslgnors to Charles Pfizer & Company, Brooklyn, N. Y., a, corporation of New Jersey No Drawing. Application August 5, 1939,

- Serial No. 288,570

6 Claims.

'from gluconic acid by means of an oxidative fer-' mentation Bcrnhauer and Giirlich, Biochem. Z. 280: 367-721, 1935) However, the action of the microorganisms employed appears to be specific, and there is'no evidence that this method is applicable to other aldonic acids. In so far as we have been able to determine, there is no record of the production of any 2-keto-aldonic acid in any substantially non-aqueous solvent. Since the molecule of an aldonic acid contains many points of attack, it is obvious that chemical agents must be highly selective in their action if they are to dehydrogenate only the second carbon atom.

It is disclosed in our copending application Serial No. 247,644, filed December 24, 1938 (now Patent No. 2,188,777), that chlorates acting upon aldonates dissolved in a mildly acid aqueous medium in the presence of a vanadium compound as catalyst, will produce the desired result, but that if the free aldonic acids are used, the reaction in aqueous medium is not satisfactory. Esters and lactones in water obviously are likewise unsuitable since they soon give the acid by hydrolysis.

We have now found that when an aldonic acid, or a lactone or ester of an aldonic acid is treated with a substantially non-aqueous lower alcohol, such as methanol or ethanol, and a chlorate in the presence of a small amount of a vanadium catalyst, such as vanadium pentoxide, dehydrogenation of the second carbon atom takes place .and the ester of the corresponding 2-keto-aldonic acid is obtained in good yield. Vanadium compounds of lower states of oxidation may be used if desired, since the oxidizing action of the chicrate immediately converts them to the pentavalent state. It is advantageous to add a small amount of a mineral acid, such as phosphoric acid or sulfuric acid, to promote both esterification and oxidation; The amount of acidifying agent is preferably only a small fraction of the amount of aldonic acid present.

Our dehydrogenation process is applicable to the aldonic acids, their lactones and esters, in general, and in fact has been successfully applied not only to those containing six carbon atoms, but also to aldonic acids of five and seven carbon atoms, respectively. A particular advantage of our process is that the esters can be readily isolated by crystallization without preliminary removal of .the added mineral acid and the dis solved catalyst.

When methyl alcohol is used as the organic solvent, a sufficient amount of the vanadium catalyst dissolves to permit observation of the completion of the action by the change from yellow to the blue of the vanadyl compounds. When higher alcohols are used, the vanadium catalyst is so nearly insoluble that the; color change is not readily recognizable, but the process is still operative, although more slowly.

The temperature of the reaction is limited only by practical considerations of economical operation. The dehydrogenation proceeds satisfactorily at the boiling point of the solvent, but we prefer to work at room temperature, since the results are in every case at least as good as at higher or lower temperatures.

Example 1.Preparation of the methyl esterof 2-keto-gluconic acid from d-glucono-y-lactone. 17.8 grams of glucono- -lactone was refluxed in 100 cc. of 99.9% methyl alcohol, in the presence of 1 cc. 85% phosphoric acid until solution was efiected. After dissolving 3.7 grams of sodium chlorate in the clear solution, the mixture was allowed to cool. Two grams of vanadium pentoxide was then added and the mixture was shaken for about four days at room temperature Completion of the oxidative process was evidenced by the color change from yellow to the green-blue of the vanadyl ion. The unreacted solid catalyst was removed by filtration.

The filtrate was concentrated under diminished pressure and inoculated with crystals of 2-keto-d-gluconic methyl ester. The crystalline ester, contaminated with a little sodium chloride, was separated by filtration. lization from dioxan deposited pure 2-keto-dgluconic methyl ester. The melting point 173- 175 C. and rotation (.0 3: 768 in water (0, 2.8) agrees with the values given .by Ohle (Ber. vol. 63, p. 849 (1930)).

To measure the 2-keto acid produced, an aliquot portion of the original filtrate was treated with suflicient calcium ferrocyanide to precipitate the vanadyl ion. After removal of the latter precipitate, the keto acid was determined by the usual procedure using F'ehlings reduction. -It indicated a 55% yield as 2-keto. acid.

Example 2.Preparation of themethyl ester of 2-keto-gluconic acid from d-glucono-t-lactone. 36 grams of d-glucono-a-lactone was dissolved in A recrystalgluconic methyl 200 cc. methyl alcohol by refluxina in the presence oi 2 cc. of 85% phosphoric acid. '11; this solution was added 7.2 grams of sodium chlorate and 2 grams oi adium pentoxlde, and the mixture was shaken at room temperature for about five days until the oxidation was completed. The product was assayed as in Example 1 and showed a 60% yield oi 2-keto-gluconic methyl ester. The oxidized solution was filtered and evaporated under reduced pressure and the ester allowed to crystallize. The crystalline 2-ketoester was recrystallized from diomn to remove a little sodium chloride. The

librium melting point was then I'm-176 C. and the equiin water (c, 2.8) was 'l6.8.

' when practically all the ester was crystallized from the reaction mixture, the solution was hydrolyzed with water and then treated with calcium carbonate. The calcium phosphate was separated by filtration and a substantial quantity of the unreacted gluconic acid was recovered in the form of its calcium salt.

Example 3.-Oxidation of l-idonic acid to the methyl ester of -2-keto-l-idonic acid. A solution of 19.6 grams of idonic acid in methyl alcohol containing 2 cc. of 85% phosphoric acid was treated with 3.6 grams of sodium chlorate and 2 grams of vanadium pentoxide. The mixture was shakenat room temperature for about a Week when the reaction was completed. An anaylsis by Fehlings reduction indicated a 50-60% yield of 2-ke'to-idonic acid. The catalyst was separated, and the reaction mixture evaporated to about cc.under diminished pressure. A little ether was added to the concentrated alcoholic solution to facilitate crystallization of the 2-ketoidonic methyl ester. The latter was recrystallized from dioxan. The melting point 155-156 C. and the rotation in methyl alcohol (0, 1) are in good agreement with the values given by Reichstein and Gruessner (Helv. Chim. Acta 17: 325).

Example 4.-The oxidation of l-gulono-lactone to the methyl ester of Z-keto-l-gulonic acid. 36 grams of l-gulono-lactone was suspended in 300 cc. of methyl alcohol containing 2 cc. of 85% phosphoric acid, 7.2 grams of sodium chlorate, and 2 grams of vanadium pentoxide. The mixture was shaken for several days and then filtered. 20 grams of the unreacted lactone was recovered along with the catalyst. The filtrate was evaporated to dryness and taken up with a little methyl alcohol, whereupon some of the sodium chloride was separated by filtration. The filtrate was then treated with a little ether to promote crystallization of the 2-keto-gulonic methyl ester. The crystalline ester was recrystallized from dioxan, and its melting point 154156 C. and

- rotation in methyl alcohol (c, 1) agree with the physical constants given for 2-keto-idonio in Example 3, and with those of Reichstein and Gruessner cited in Example 3.

Example 5.-The oxidation of l-idonic acid to the ethyl ester of 2-keto-l-idonic acid. A solution of 39 grams of idonic acid in absolute ethyl alcohol was treated with 2 cc. 85% phosphoric acid, 7.2 grams of sodium chlorate and 2 grams of vanadium pentoxide. This mixture wasshaken .ior about eight days at room temperature. When j -y-lactone.

the reaction was completed. the catalyst was removed by nitration and the filtrate analyzed by llehlincs reduction. The yi ld r -weidonic aster is-%. when this same oxidation is performed in methyl alcohol, the yield is about twice as high.

Example 6.--Dehydrogenation oi d-mannano- 36 grams of d-mannano-v-lactone was dissolved in 250 cc. methyl alcohol with 3 cc. phosphoric acid and 7.2 grams of sodium chlorate under reflux. On cooling, some lactone crystallized; however, 2 grams of vanadium pentoxide was added and the suspension shaken for four days. When the reaction was completed the catalyst and some unreacted lactone were separated by filtration. A. Fehling's reduction determination showed a 45% yield of 2-keto ester.

The filtrate was evaporated under diminished pressure and inoculated with 2-keto-gluconic methyl ester. The crystalline ester contaminated with a little sodium chloride was recrystallized from dioxan. Its physical constants are identical with those obtained from the oxidation oi. d-gluconic lactone as described in Examples 1 and 2.

Example 7.The oxidation of d-gluconoalactone to the ethyl ester of Z-keto-gluconic acid.

86.6 grams of glucono-e-lactone was treated with 250 cc. absolute ethyl alcohol and 3 cc. of 85% phosphoric acid. This was refluxed until the lactone dissolved. The cooled solution was then treated with 7.2 grams of sodium chlorate and 2 the syrup and in this way the ethyl ester of the 2-keto acid was converted to the methyl ester. This crystalline ester was separated by adding a little ether and subsequently recrystallized from dioxan. Its values for the melting point and rotation are in agreement with those in Examples 1 and 2, and with those of Ohle cited in Example 1.

Example 8.-Oxidation of l-galactonic acid to The reaction a the methyl ester of 2-keto-galactonic acid. A K

solution containing about 35 grams of l-galactonic acid in methyl alcohol was treated with 2 cc. of 85% phosphoric acid, 7.2 grams of sodium chlorate and 2 grams of vanadium pentornde. The mixture was shaken for about three days, whereupon the catalyst was removed by filtration and the filtrate analyzed by Fehlings reduction. The yield of 2-keto acid was about 25%.

Example 9.--Dehydrogenation of glucono-tlactone to form the methyl ester of 2-keto-gluconic acid at elevated temperatures. of glucono-o-lactone was dissolved in 250 cc. of methyl alcohol and 3 cc. of 85% phosphoric acid on a reflux. When the lactone dissolved, 14.8 grams of sodium chlorate and 1 gram of vanadium pentoxide were added. After about two hours heating at a boil under reflux, the color change indicated completion of the reaction, whereupon the catalyst was removed by filtration. The filtrate analyzed about 35-40% 2-keto acid. It was 72 grams evaporated under diminished pressure and the 2-keto-gluconic methyl ester readily crystallized. The ester was recrystallized from dioxan, and the values for the physical constants are identical with those given in Examples 1 and 2.

The invention claimed is:

1. Process for preparing esters of 2-keto-aldonic acids which comprises treating a member selected from the group consisting of the aldonic acids, their lactones and esters with a substantially non-aqueous lower alcohol containing a minor amount of a nonoxidizable acid selected from the group consisting of phosphoric acid and sulfuric acid and a chlorate in the presence of v jecting said substance to the action of a chlorate and a vanadium catalyst in a lower alcohol containing a minor amount of a nonoxidizable acid selected from the group consisting of phosphoric acid and sulfuric acid.

4. Process for the preparation of esters of 2- keto-idonic acid by the dehydrogenating action of sodium chlorate and a-vanadium catalyst upon idonic acid in a medium consisting of a lower alcohol acidified with a nonoxidizable acid selected from the group consisting of phosphoric acid and sulfuric acid.

5. Process for the preparation of esters of 2- keto-gluconic acid by the dehydrogenating action of sodium chlorate and a vanadium catalyst upon gluconic acid in a medium consisting of a lower alcohol acidified with a nonoxidizable acid selected from the group consisting of phosphoric acid and sulfuric acid.

6. Process for the preparation of esters of 2- keto-gulonic acid by the dehydrogenating action of sodium chlorate and a vanadium catalyst upon gulonic acid in a medium consisting of a lower alcohol acidified with a nonoxidizable acid se-" lected from the group consisting of phosphoric acid and sulfuric acid.

RICHARD PASTERNACK. PETER P. REGNA. 

